1. Field of the Invention
The present invention relates to a fuel cell stack including a stacked structure composed of a plurality of electricity-generating cells each having a membrane electrode assembly with a pair of electrodes arranged on both sides of the electrolyte, the membrane electrode assembly being sandwiched by separators.
2. Description of the Related Art
In general, a solid polymer electrolyte fuel cell constitutes an electricity-generating cell in which an electrolytic membrane electrode assembly (membrane electrode assembly) are sandwiched by separators, the electrolytic membrane electrode assembly having an anode side electrode and a cathode side electrode opposed on both sides of an electrolytic membrane (electrolyte) of a high-polymer ion-exchange membrane (cation exchange membrane). Such a kind of electricity generating cell is used as a fuel cell stack by alternately stacking a prescribed number of the electrolytic membrane electrode assemblies and separators.
In this electricity-generating cell, the fuel gas supplied to the anode side electrode, e.g. gas containing mainly hydrogen (hereinafter also referred to as “hydrogen-contained gas”) is ionized on an electrode catalyst, and the ionized gas is moved to the cathode side electrode through an electrolyte. The electrons produced during said processes are extracted by an external circuit so as to be used in direct electric energy form. Additionally, since an oxidizer gas, for instance, gas containing mainly oxygen or air (hereinafter referred to as “oxygen-containing gas”) is supplied to the cathode side electrode, water can be produced under the reaction of hydrogen ions, electrons and oxygen with one another.
Meanwhile, the fuel cell stack includes electricity-generating cells that are likely to cause a temperature reduction due to the external heat dissipation in comparison of other electricity-generating cells. For example, the electricity-generating cell (hereinafter referred to as an end cell) located at a distal end in a stacking direction shows a remarkable temperature reduction owing to large heat dissipation from a power extracting plate (collector) for collecting charges created by the respective electricity-generating cells, or an end plate which is provided for holding the stacked electricity-generating cells.
As regards the fuel cell stack, the following drawback has been reported. Namely, owing to such temperature reduction, the distal end cell is likely to cause condensation as compared with the electricity-generating cells of which location are in the central portion of the fuel electrode stack, which ends up deterioration of discharging of the produced water, and as the result said temperature reduction deteriorates the electricity-generating performance. Particularly, there is a problem such that when the fuel cell stack starts up in an environment below the temperature of freezing, the water produced at the end cell might be frozen so that the temperature of the distal end cell cannot be risen, thereby leading to the voltage drop.
In order to overcome this kind of drawback, a solid-state polymer electrolyte fuel cell has been proposed which is provided with an end cell 1 as shown in FIG. 8. In the end cell 1, an electrolytic membrane electrode assembly 2 is sandwiched by separators 3 and 4. The electrolytic membrane electrode assembly 2 has a fuel electrode 2b and an oxidizer electrode 2c placed on both sides of a PE membrane 2a. The separator 3 has a groove 3a for feeding a fuel gas, which is located on the one surface opposite to the fuel electrode 2b, as well as a groove 3b for coolant, which is located on the surface opposite to the one surface.
The outer separator 4 constituting the end cell 1 has a groove 4a for feeding the fuel gas, which is located on the one surface opposite to the fuel electrode 2b, however, it does not have a groove for coolant located on the surface opposite to the one surface as the separator 3 has. Thus, the separator 4 adopts the structure which is designed not to be cooled so much that the end cell 1 can be prevented from being excessively cooled by the coolant.
[Patent Reference No. 1]
JP-A-8-130028 (paragraphs [0053]-[0055], FIG. 9)
As described above, Patent Reference No. 1 discloses a structure to warm up the entire electricity-generating portion through self-heating caused by electric generation of the fuel cell, which can also prevent the condensation at the end cell 1 as the result of excessive cooling caused by the coolant.
However, particularly, in the case where the fuel cell is actuated in an environment below the temperature of freezing, the cell temperature must be raised rapidly to the temperature where the water is produced without being frozen. However, the structure disclosed in Patent Reference No. 1 is not enough to cope with such a situation.
Specifically, in the case where the fuel cell is actuated in an environment below the temperature of freezing, blocking the path for a reactive gas flow, which is caused by the produced water being frozen, is likely to occur within the electrodes that is constituting the electrolytic membrane electrode assembly. In this case, the reactive gas diffusing path is blocked, whereby the cell voltage drop is induced. In order to overcome such a drawback, temperature of the electrode must be swiftly raised to 0° C. or higher. However, it is not possible for the structure disclosed in Patent Reference No. 1 to maintain the electrode temperature at 0° C. or higher. This brings about another problem such that self-heating of the cell cannot be functioning any more due to said abrupt voltage drop of the end cell.